Currently, an apparatus based on optical coherence tomography (OCT) (i.e., an optical coherence tomographic imaging apparatus; hereinafter referred to as an OCT apparatus) which uses a low coherence interferometer can obtain tomography images of a specimen (test object) at high resolution. The OCT apparatus is becoming indispensable as an ophthalmologic apparatus for specialized retinal outpatient clinics. Also, various developments are being carried out toward application to endoscopy.
By irradiating a test object with measuring beam which is a low coherence light, the OCT apparatus can measure backscatter from the test object at high sensitivity using an interferometer.
Also, by scanning the test object with the measuring beam, the OCT apparatus can obtain tomography images at high resolution.
The OCT apparatus can also acquire tomography images of the retina in the ocular fundus of the eye under examination at high resolution, and is widely used for ophthalmologic diagnosis of the retina.
In particular, with the advent of the OCT apparatus, tomography images of the retina can now be acquired at high resolution, enabling detection of small lesions which have been difficult to find by conventional diagnosis procedures.
Therefore, the OCT apparatus is expected to facilitate early detection of ocular fundus diseases, and various developments are being carried out.
The OCT apparatus, when used in ophthalmologic diagnosis of the retina, uses near-infrared light, which is poorly absorbed by water. Moreover, shorter wavelengths which are more poorly absorbed are favored, and near-infrared light with a center wavelength of around 800 nm is widely used conventionally. The near-infrared light around 800 nm is strongly absorbed or scattered in deep layers of the retina. This makes it difficult for light to reach the choroid, raising a problem in that it is difficult to acquire tomography images of the deep layers.
To deal with this problem, Japanese Patent Application Laid-Open No. 2007-181631 discloses an OCT apparatus which uses a wavelength longer than 800 nm with a larger penetration depth to carry out imaging at deep sites near the choroid in the back of the eye. The OCT apparatus disclosed in Japanese Patent Application Laid-Open No. 2007-181631 is an ocular fundus imaging apparatus which performs OCT using a light source with a wavelength of 1000 nm to 1100 nm to image deep layers of the retina including the choroid and sclera.
Also, Japanese Patent Application Laid-Open No. 2007-151631 discloses ophthalmologic radiography apparatus which uses a light source with a wavelength in the 1,500 nm band in addition to light in the 800 nm band and changes focusing position with the wavelength to increase depth of focus. The ophthalmologic radiography apparatus makes lights from the two light sources enter a test object alternately, combines tomography images obtained alternately, and thereby obtains a tomography image with a wide range.